Last summer's vote by members of the International Astronomical Union elevated Ceres from being merely the largest member of the asteroid belt to a prime candidate for "dwarf planet" status. And from what astronomers have learned about it recently, Ceres is making a good case for that promotion.The most recent insights come from a trio of astronomers led by Andrew S. Rivkin (Applied Physics Laboratory), who examined the big asteroid's near-infrared spectral signature last year. Previous work had already established the presence of clay-like minerals that include water as part of their molecular structure. Rivkin's team has used the body's infrared fingerprint to refine the kinds of materials that might lie on its surface. The best candidates, he reported at a recent meeting of planetary scientists, are iron-rich clays that contain roughly 5% carbonates — just the kind of minerals that would form on what was once a wet surface.

A team of astronomers led by Benoit Carry of the Paris-Meudon Observatory used state-of-the-art adaptive optics instrumentation available at the Keck observatory, Mauna Kea, Hawaii, to image the surface of Ceres with a spatial resolution of ~30km. The observations were carried out during the September 2002 opposition of Ceres in the near-infrared J/H/K-bands, at a wavelength range particularly well adapted to investigate the composition and properties of planetary surfaces. The team produced albedo maps covering 80 percent of the asteroid, which appears to display a wealth of 40 to 160km large geological features with intensity in reflected light varying by ~12 percent across the surface. The team suggests that the variations could be due to terrain features, as well as differences in their surface composition and/or degree of alteration by space weathering effects (such as aging of surface due to interaction of solar wind, micrometeorites impacts, etc).

The team also derived measurements of the dimension and shape of the dwarf planet Ceres, which can be considered as an oblate spheroid of radii a=481km and b=447km (incertitude of ±14 km). The direction of its spin axis in a J2000 coordinate referential is right ascension=287 degrees and declination=69 degrees (5 degrees of incertitude). These two results are in agreements with earlier reports made by Thomas et al. (Nature 2005) from the analysis of the Hubble Space Telescope observations.

The surface of the solar system's largest asteroid, Ceres, has been mapped in infrared light in fine detail for the first time. The feat will pave the way for a better determination of the surface composition of Ceres, whose interior is believed to be 25% water ice.

Expand (61kb, 560 x 529)A false-colour infrared map of Ceres shows bright regions in red and dark regions in blue. The circular blue feature near the centre may be an impact crater, while the nature of the reddish patches is unclear. Black patches at the poles are due to a lack of data Credit B Carry/C Dumas et al./Keck

Last month, the asteroid was re-classified as a dwarf planet by the International Astronomical Union (IAU), and now new images of its surface reveal a surprisingly diverse surface terrain, scientists say.

"We thought Ceres had a flat surface, but our images show that it is rich in surface features" - Benoit Carry, Observatoire Paris-Meudon.

Carry's team has produced 360 infrared images of Ceres while observing it in rotation at the Keck observatory in Mauna Kea.

"We think Ceres still contains pristine water from when the Solar System was formed" - Christophe Dumas, Eso

The results were presented here at the Division of Planetary Sciences meeting in Pasadena, California.

The four Hubble images of Ceres were taken over a 2-hour and 20-minute span, the time it takes the object to complete one quarter of a rotation. One day on Ceres lasts 9 hours. The bright spot that appears in each image is a mystery.

The largest known asteroid could contain more fresh water than Earth and looks like our planet in other ways, according to a new study that further blurs the line between planets and large space rocks.Astronomers took 267 images of asteroid Ceres using the Hubble Space Telescope. From these images and subsequent computer simulations, they suggest Ceres may have a rocky inner core and a thin, dusty outer crust.

A team led by Peter Thomas of Cornell University said that Ceres is nearly spherical, which suggests that gravity controls its shape. Also, the asteroid's non-uniform shape indicates that material is not evenly distributed throughout the inside. These and other new clues, including Ceres' low density, point to an interior loaded with frozen water. The results are detailed in the September 8th 2005 issue of the journal Nature.Ceres has long been considered one of the tens of thousands of asteroids that make up the asteroid belt between Mars and Jupiter. At 930 km in diameter , about the size of France, it's the largest asteroid in the belt, accounting for about 25% of the belt's total mass.Astronomers had thought Ceres might never have been heated enough to create layers of material.But new computer models now suggest Ceres has a differentiated interior – dense material in the core and lighter stuff near the surface. Possible configurations include a mantle rich in water ice around a rocky core. If this mantle is composed of at least 25% water, Ceres would have more fresh water than Earth, according to a statement released by the Space Telescope Science Institute, which operates Hubble for NASA and the European Space Agency.

"The most likely scenario from the knowledge we have on how other objects form, it probably has a rocky core and a mantle. That mantle is probably some watery, icy mix, with other dirt and constituents. That mantle could be as much as ¼ of the whole object. Even though it's a small object compared to Earth, there could be a lot of water" - Joel Parker, study co-author, Southwest Research Institute.

On Earth, fresh water makes up only a thin layer just a few miles deep in some places, less in others. The water layer proposed for Ceres, while smaller in circumference, is many miles thicker.The total volume of water on Earth is about 1.4 billion cubic kilometres, around 41 million of which is fresh water. If Ceres' mantle accounts for 25 percent of the asteroids mass, that would translate to an upper limit of 200 million cubic kilometres of water.Since all the nine "regular" planets have differentiated interiors, this new view of Ceres has some astronomers calling Ceres a "mini-planet," adding fuel to an ongoing debate over exactly what qualifies as a planet. Other researchers recently announced the discovery of 2003 UB313, a round object in our solar system 1-1/2 times larger than Pluto and about three times further away from the Sun. But even an object of this size – at 2,100 miles in diameter roughly four times the size of Ceres – doesn't receive universal endorsement as being a planet.

One astronomer, Brian Marsden, who runs the Minor Planet Centre where data on small bodies is collected, says that if Pluto is considered a planet, then any other round worlds should also be considered planets. Under this definition, which some other astronomers subscribe to, Ceres, 2003 UB313 and a handful of other large objects would be named planets. The alternative is to stop calling Pluto a planet.

Another explanation is that Ceres is a sort of 'baby' planet – an underdeveloped version of Earth and other rocky planets. Looked at this way, Ceres appears as other fledgling planets might have looked more than 4 billion years ago. The leading theory for planet formation holds that small rocks collided, stuck and gradually grew. Depending on location and orbit, a developing world may or may not have encountered enough raw materials to become as large as the four traditional rocky planets.

"Ceres is an embryonic planet. Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet" - Lucy McFadden, observation team member, Department of Astronomy at the University of Maryland.

In 2015 scientists will get a close up look at Ceres when the NASA Dawn mission orbits the asteroid. A closer look should provide more clues about the asteroid's composition.